Observations of gravitational waves (GWs) generated by binary black hole (BBH) mergers provide us with a powerful way to explore the strong and highly dynamical regime of gravity theories. The ringdown of the BBH merger, consisting of a series of quasinormal modes (QNMs), is of particular interest for both the black hole (BH) spectroscopy and the inspiral-merger-ringdown consistency check. Unlike the QNM frequencies that only depend on the properties of the remnant BH, the excitation amplitudes and phases of QNMs depend on the progenitor system, and calculating them is beyond the perturbative approach. In this paper, by performing self-consistent fully nonlinear simulations of a BBH merger in shift-symmetric scalar-Gauss-Bonnet (sGB) gravity as well as in sGB gravity allowing for scalarization, and extracting the QNM excitation, we explore the possible deviations from general relativity at the ringdown stage. We numerically verify that the mode frequencies are consistent with the theory prediction, and provide the fitting results of mode amplitudes and phases. We find relatively small changes in the mode excitation, considering that the largest coupling we used in the simulations is close to the limit of loss of hyperbolicity. To demonstrate that our results are robust against the eccentricity caused by the imperfect initial data, we also perform eccentricity reduction and estimate the effect caused by the initial eccentricity. These studies are useful for understanding the ringdown in sGB gravity.

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• Alternative gravity theories
• Gravitational waves
• Numerical relativity